The separation of dusts from industrial-gas streams often is done by using filters made of natural or synthetic fibers. It is common to shape the filter elements as bags and place them in structural enclosures called baghouses. Besides providing support for the filter elements, baghouses contain baffles which direct air-flow into and out of the filter elements. Also, the baghouses include equipment for cleaning the filter bags and means for collecting and discharging the dust removed from the filter bags.
Baghouses are used when dust-laden air is to be discharged to the atmosphere or when valuable particulate matter is to be recovered from a process venting system. The sizes of baghouses may range from small bin-venting filters to large multicompartment filters that receive dust-laden air from extensive systems of exhaust ducts.
Dust filtration in baghouses is accomplished by passing the dust-laden air through a filter fabric that is in the form of a cylindrical tube or an oblong bag. As the air passes through the fabric, the dust is retained on the surface and in the interstices of the fabric. A filter cake builds up which also acts as a filter medium. The filter cake is dislodged periodically. Otherwise, the build-up of filter cake would cut off air flow through the filter bag. p In the past, filter fabrics have been selected based upon various properties judged to be important for the particular applications by the designers of the pollution control systems. Among the properties considered are:
permeability-the fabric should be sufficiently porous to permit the desired flow of air, PA1 mechanical strength-the fabric should resist the tensile forces caused by operating pressure differentials, by mechanical shaking during cleaning, and by pulsing during reverse air-flow and the fabric should withstand abrasion when it is clamped to and supported by the housing structure and when it is subjected to the impact of the filtered material, PA1 corrosion resistance-the fabric should resist attack and weakening due to chemical action caused by the filtered materials, especially if moisture is present due to condensation, PA1 heat resistance-for some processes, the fabric is exposed to high-temperature exhaust gases, PA1 cleanability-the fabric should have a surface texture that is conducive to rapid release of the filter cake during cleaning and the fabric also should have a high rate of electrostatic-charge dissipation so as to shed charged dust particles, PA1 dimensional stability-the fabric should resist stretching or shrinking that would affect its permeability.
Various filter fabrics have been suggested and used in the past. In one commonly used arrangement, a microporous teflon sheeting is secured to a suitable substrate. A limitation of this and other similar filters using microporous materials is that the permeability of the filter is established as the microporous material is manufactured. Consequently, there is limited, if any, flexibility in selecting the desired porosity of the filter as it is fabricated.
Another problem with filters which are in use currently is the tendency to build up and retain electrostatic charge. Although consideration is given, in the selection of materials used as filter fabrics, to the rate of dissipation of electrosatic charge when the filters are being cleaned, the filters are not arranged to prevent entirely or reduce significantly the tendency or building up large electrostatic charges. Consequently, there remains the hazzard to those who clean such filters or maintain the baghouse of highly charged filters.